Data AcQuisition And Real-Time Analysis
Scope - Spectrum - Spectrogram - Signal Generator
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Sound Card Spectrum Y-log Toggle (Power Spectrum)

Controls: Spectrum Dialog >> Y-log
Macro: Ylog

This button controls whether the Spectrum display will show linear (magnitude) or logarithmic (power) spectra. You can also use ALT+Y to toggle this state, even when the dialog is not visible.

For convenience, there are duplicates of this button at the tops of the Curves and Limits dialogs.

Since many spectra can have a wide dynamic range, the logarithmic mode is usually preferred. When it is active, the Y axis is shown in dB. On an uncalibrated system the dB reference defaults to full scale, which means the top of the Y axis will be 0 dB, with negative values below it.

There are 21 basic Y-log dB ranges, controlled by the same display magnification controls as other modes: The small arrow buttons above the Y axis, or dragging the Y axis, or PgUp and PgDn keys. The widest range is 0 to -180 dB and the narrowest is 0 to -6 dB. The default range is 0 to -90 dB.

You can offset the range using SHIFT with the PgUp/PgDn keys. For example, if you shift the default range so that the top is -20 instead of 0, the bottom will be -110 dB. SHIFT+Home will restore the original unshhifted range top.

Using these operations together, you can magnify any portion of a spectrum. For example, if you are on the default range and you want to look at just the portion between -60 and -66 dB, you could use SHIFT+PgUp to set the top of the Y axis to -60 dB, then use unshifted PgDn to set the bottom of the axis to -66 dB.

The normal dynamic range of a 16-bit input is theoretically only about 96 dB, but that is the ratio of the largest signal to the total noise in all frequency bands. The contribution from each individual frequency (the "noise floor") can be much lower... in theory, as low as -125 dB for an FFT of 1024 points, assuming "infinite" precision in computations (no roundoff or truncation).

Perhaps surprisingly, it is possible for a spectrum to have components larger than the full-scale range. For example, the fundamental component of a square wave is about 27% larger than the peak-to-peak waveform. (See Making Waves via Sine Wave Synthesis.) To handle cases such as this, each log range has extra room above 0 dB adequate for worst-case conditions.

Since the dB is a unit of power (proportional to the square of magnitude), certain relationships are more easily seen. Also, the entire range of the signal can be shown in one display. Compare that to the linear magnitude display, where if the magnification is set so that you can see the higher peaks of a typical signal, the lower peaks and the background noise are usually off the bottom of the trace.

In acoustics and electronics, a logarithmic vertical display is the standard means for showing the frequency response of a system. Because of the nature of logarithms, the response to different input levels gives the same shape with only a vertical shift, so it's easy to see if two responses are the same or different. With a linear magnitude display, this is much harder, since the two responses would be related by a multiplicative factor.

Macro Notes:

Ylog=1 sets Y-log mode, Ylog=0 restores linear magnitude mode, and Ylog=x toggles between them.

See also Spectrum Control Dialog


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